Composite materials (or simply “composites”) are lightweight and high strength and thus are widely used to fabricate composite structures, such as aircraft components including airflow ducts, and other types of structures. As known in the art, composites are manufactured from a reinforcement material embedded in a matrix material. Acoustic attenuation features can be added to the composite structures, resulting in “acoustically-treated composite structures.”
Conventional methods for fabricating acoustically-treated composite structures require two separate female molds for forming composite ply lay-ups. The lay-ups are vacuum bagged, cured separately, and then joined together by fasteners and bonding agents forming seams where joined. An acoustic core and perforated acoustic facesheet are included in one or both of the composite ply lay-ups to provide acoustic treatment for sound attenuation. A third tool is typically used to align the two cured products before fastening and bonding them together at the seams. The conventional acoustically-treated composite structure is layed up from the outside in. Thus, as the interior surface of the conventionally fabricated acoustically-treated composite structure is opposite a tool side, the resultant interior surface is typically rough and not closely toleranced. Therefore, post-curing processing of the interior surface may be needed when the interior surface thereof is a critical feature such as, for example, in an acoustically-treated composite airflow duct.
An airflow duct provides a compressor of an aircraft auxiliary power unit (APU) with a uniform supply of air in order to prevent the compressor from stalling. The interior surface of the airflow duct serves as the flowpath for air passage therethrough. Even small interior surface discontinuities can cause airflow distortion problems that can result in engine problems as well as significant efficiency losses. Therefore, if a conventionally fabricated acoustically-treated composite airflow duct is to retain its function of providing sufficient air with a minimum of turbulence, the above-described post-curing processing is often required to smooth the interior surface thereof to create as little distortion as possible.
Therefore, conventional methods for fabricating acoustically-treated composite structures require significant time-consuming assembly and post-curing processing with extensive tooling. Maintaining the positioning of acoustic cores and perforated acoustic facesheets during vacuum bagging is difficult. Conventional fabrication methods also result in an expensive and heavy acoustically-treated composite structure as the seams and fasteners undesirably add weight. The presence of seams in the fabricated acoustically-treated composite structure also compromises its structural integrity, and aerodynamic and fireproof properties. Monolithic composite structures have been fabricated, but inclusion of sound-attenuating acoustic cores and facesheets therein has not been successful because of the complex shape and relatively small cross-sectional sizes used in some structures, for example aircraft airflow ducts. As used herein, the term “monolithic” refers to a seamless unitary closed cross-sectional structure. The term “closed” as used herein refers to a structure that has a continuous cross-section.
Accordingly, it is desirable to provide monolithic acoustically-treated composite structures and methods for fabricating the same. In addition, it is desirable to provide monolithic acoustically-treated composite structures that have improved structural integrity, improved aerodynamic and fireproof properties, and are more lightweight than conventional acoustically-treated composite structures. It is also desirable to provide methods for fabricating the same that are simpler, eliminating seams, requiring fewer tools, reducing assembly and post-curing processes, permitting more control over the critical tolerances of the interior surface, and providing easier inclusion of sound attenuating acoustic cores and perforated acoustic facesheets.